The present invention relates to a battery apparatus comprising an electricity storing device having a battery, such as a secondary battery, and a control system therefor.
In recent years, various kinds of electric energy storing devices, such as a secondary battery and a capacitor (hereinafter, referred to simply as a battery) have been widely used as a clean energy source for a vehicle. However, it is sometimes difficult to increase the voltage or power capacity of a battery, and so a plurality of unit batteries are often used by connecting them in series or in parallel.
In a case of connecting a plurality of batteries in series, when the batteries individually vary in electric power capacity, initial voltage or temperature, it is difficult to uniformly share voltage among the batteries. Particularly, in a case where lithium secondary batteries or electric double layer capacitors using an organic solvent as the electrolyte are connected in series, a variation in terminal voltage of the batteries causes over-discharge to occur, resulting in degradation of performance and shortening of the life of the batteries.
In the case of lithium secondary batteries, a protective function is typically provided to stop charging or discharging of the batteries when an over-charge or over-discharge condition of the secondary batteries is detected. In a case where a plurality of lithium secondary batteries having such a protective function are connected in series, the safety of the batteries can be secured by stopping the charging at a time when the voltage of some of the secondary batteries which have a high initial voltage, reaches an over-charge protective level. However, charging of the remaining secondary batteries, which have a low initial voltage, is stopped in this case in the middle of charging before they have been charged sufficiently. Similarly, discharging is stopped at the time when the voltage of the secondary batteries, which have a low initial voltage, reaches an over-discharge level; and, in this case as well, the discharging of the secondary batteries, which have a high initial voltage, is stopped while they still store a sufficient quantity of electricity.
As described above, when a plurality of batteries having a protective function to prevent over-charge and over-discharge of the batteries are connected in series, the available electric capacity is only a part of the electric power capacity which may be obtainable with all of the plurality of batteries, and, accordingly, the availability of the electric power is reduced. In order to avoid such a problem, it is necessary to take measures to ensure that an initial voltage of each of the batteries connected in series is accurately detected, and the voltages of all the batteries are made uniform when the initial voltages of some of the batteries are different from the initial voltage of the other batteries.
Further, there has been a recent trend in which lithium secondary batteries, nickel-metal hydride secondary batteries, and electric double layer capacitors are used for storing electricity for an electric vehicle. However, in any type of these batteries, it is requited to accurately measure the quantity of electricity stored in the batteries. In measuring the quantity of stored electricity (hereinafter, referred to as a remaining quantity measurement), the terminal voltage of a battery is measured and a remaining quantity of charge is estimated from the terminal voltage and an SOC (state of charge; percentage of remaining quantity on the basis of a full charge of 100%). In the estimation, a highly accurate calculation is performed by taking into consideration a change in internal resistance of the battery due to the temperature or age of the battery. In such a remaining quantity measurement, the voltage of the battery must be measured with a high accuracy of several tens of mV.
An example of a method of detecting the voltage of batteries connected in series is disclosed in Japanese Patent Application Laid-Open No.10-191573. In the battery charging system using this conventional technology, a secondary battery group is composed of a plurality (three, in this example) of secondary batteries connected in series. A discharging circuit is connected in parallel to each of the secondary batteries. A switching circuit is provided. The switching circuit selects one set out of three sets of input terminals, each set being composed of two input terminals, to connect to the output terminals, and each of the three sets of input terminals is connected in parallel to each of the secondary batteries. Input terminals of a differential amplifier are connected to the output terminals of the switching circuit, and output terminals of the differential amplifier are connected to analogue input terminals of a micro-controller.
In this conventional system, the micro-controller outputs a signal to select a +terminal and -terminal of a secondary battery using the switching circuit. A voltage of the secondary battery transmitted to the differential amplifier through the switching circuit is transmitted to the micro-controller from the differential amplifier. The micro-controller then successively outputs a signal to the switching circuit to select the next secondary battery. The voltages of the other secondary batteries are successively selected by the switching circuit in a similar manner and the values thereof are read and stored in the micro-controller. The micro-controller turns on a switch of the discharging circuit corresponding to a secondary battery having a maximum terminal voltage to effect a discharge thereof, and controls the discharging so that the voltage of the secondary battery becomes equal to the voltage of the other secondary batteries.
In the above-mentioned conventional system, there are variations in resistances of a plurality of resistors provided in the differential amplifier. Even if the variations in the resistances are approximately .+-.1%, an error in voltage becomes several hundreds of mV because the error in voltage detecting becomes larger as the level of the secondary battery is higher. Taking lithium secondary batteries as an example, the usable voltage range is between 2.7 V to 4.2 V, and the relationship between the voltage and the SOC is not linear. When there is a voltage error of several hundreds mV within an SOC range of 70 to 100%, the remaining charge quantity is measured with an error of several tens %. Therefore, the voltage error is preferably smaller than several tens of mV. The same can be said for nickel-metal hydride batteries. A differential amplifier ensuring variations of resistances less than .+-.1% is available on the market, but is very expensive.
Further, the secondary battery has an internal impedance, and, in the case of a lithium secondary battery, the internal impedance is capacitive in a low frequency range up to several kHz and inductive in frequency range above that. Therefore, when a charge current or discharge current flowing in the secondary battery varies with time (called a current ripple) or contains a disturbance, such as a surge current, the voltage of the battery contains a transient oscillation component due to the effect of the high frequency component of the current. In the voltage detection of the battery, it is required to detect a value without such a transient oscillation component.
In order to accurately detect a battery voltage in the direct current manner currently in practical use, a filter for removing any oscillation component is necessary. When a filter is selected for this purpose, it is necessary to pay attention to selection of a filter having an appropriate degree of attenuation characteristic. The objects of voltage detection in a battery are measurement of the remaining quantity, protection against over-charging and over-discharging, and compensation of a voltage imbalance in which the voltage change in response to charging and discharging current is slow; and, accordingly, the voltage may be measured on a second-by-second basis (the above-mentioned transient oscillation component is not measured). It is considered from the above that a filter having an attenuation characteristic of the second order is needed, but such a filter has a large volume.
Further, since the voltage detector constantly detects the voltage of the secondary batteries, it is preferable for the electric power consumption rate of the detecting circuit is as small as possible. Although electric power consumption rates of differential amplifiers are widely spread depending on the products, there is a limitation to how much the electric power consumption rate can be reduced because the differential amplifier is an analogue circuit and requires a bias current.
A battery is a direct current power supply, but typical loads generally require alternating current power. Further, the voltage between the terminals of the common battery changes depending on the available amount of charging and discharging power. Therefore, a battery apparatus, an electric machine or a motor using a battery needs an electric power converter, such as a DC/AC converter for controlling charging and discharging, an AC/DC converter, and/or a DC/AC inverter for converting an AC voltage to DC. A conventional uninterruptible power supply apparatus is disclosed in Japanese Patent laid-open application No. 7-298516, for example. These electric power converters generally use the chopping operation of semiconductor switches and the induced voltage produced in an inductor in the converting process. Therefore, the input and output current and voltage of these electric power converters include a large amount of high frequency ripple components changing in conjunction with the chopping operation.
When a voltage and a current contain ripple components, as described above, it is difficult to detect correct values of the voltage and the current. Particularly, this is a large problem for a battery, such as a lithium secondary battery, which needs to be controlled by accurately detecting the voltage and the current thereof. Therefore, it is necessary to remove or reduce the ripple components when the copper type electric power converter and the battery are connected. On the other hand, the copper type electric power converter uses a chopping operation by semiconductor switches and the induced voltage produced in an inductor in the converting process, as described above. Therefore, the ripple components can be-reduced to a certain degree by increasing the switching frequency or increasing the inductance.
However, in actual practice, there is a limitation to the extent to which the switching frequency or the inductance can be increased. Particularly, the switching frequency of a semiconductor switch satisfying this condition becomes lower as the current increases or the voltage increases. Further, the feasible value of inductance becomes small. In addition, the semiconductor switch and the inductor are high in price, heavy in weight and large in size.